Air conditioning systems have been devised wherein a common housing contains both the evaporator and condensor units which can be mounted externally of the roof of the bus in such a way as to be connected directly into the existing duct system in the ceiling of the bus. One such system is that disclosed in U.S. Pat. No. 4,201,064, assigned to to the assignee of this invention, and which is characterized by having both the condensor and evaporator coils arranged lengthwise in a common housing and so as to extend parallel to the sidewalls of the bus and create the lowest possible design height. There are numerous reasons for positioning both the condensor and evaporator coils in a common housing, principal among them being the necessity of direct connection of the refrigerant lines from the condensor coils via an expansion valve into the evaporator coils for most efficient operation. Nevertheless, there are important factors in favor of independently housing and mounting the evaporator and condensor units so as to be readily conformable for use on different types and styles of busses and other motor vehicles. Moreover, modular construction and design of the condensor and evaporator units into separate housings achieves other beneficial results from the standpoint of replacement for existing evaporator and condensor units which are built into the motor vehicle so as not to necessitate removal of the existing units.